It is often necessary to determine an angle, such as the angle over which a wheel or axle rotates. Some types of so-called rotary encoders or shaft encoders convert an angular position or rotation of a shaft or axle into an analogue or digital code. Shaft encoders of the mechanical type may have one or more concentric rings with openings which, when such a ring rotates, may be detected by mechanical contacts. In optical shaft encoders, the openings or alternating transparent and opaque areas may be detected using a light source and a photo detector, for example. In some applications, such as bearings, optical encoders may not be practical as grease may obscure the rings. Magnetic shaft encoders typically use a ring provided with a plurality of magnets and Hall sensors capable of detecting the magnets as the ring rotates. An example of such a shaft encoder or rotation detection device is disclosed in United States patent application US 2009/315544.
The rotation detection device of US 2009/315544 has two concentric rings, each provided with magnets, and determines an absolute rotation angle based on the detected phase difference of the rings. However, this known device is incapable of measuring angles greater than 360°, as the same phase difference occurs every 360°. In order to measure greater angles, a memory would have to be provided to store a counter value which is increased after each 360° rotation. The absolute angle which can be measured with this known device is therefore limited to 360°, after which the measured angle becomes a relative angle, that is, relative to the counter value.
International patent application WO 2014/131434 discloses an angle determining device for determining an absolute angle signal of a first part rotated with respect to a second part. The angle sensor comprises a first grating ring for generating a first signal representative of a relative position of a first sensor along a corresponding ring segment of the first grating ring. The angle sensor further comprises a second grating ring for generating a second signal representative of a relative position of a second sensor along the corresponding ring segment of the second grating ring. The first plurality and the second plurality are co-prime numbers and a difference between the first plurality and the second plurality being larger than one. The angle sensor also comprises a calculator configured for calculating the absolute angle signal using a first linear combination of the first signal and the second signal.
The angle determining device of WO 2014/131434 decreases the ambiguity of the phase difference between two rings, but does not resolve this ambiguity beyond 360°.
In summary, there is a need for an absolute angle sensor which does not rely on a memory, but still is capable of providing absolute angle measurements of angles greater than 360°.